Digital signal compression is the process of encoding an analog signal, such as a video or audio signal, into a digital bit stream of "1"s and "0"s. The resulting digitized data may be stored on a compact disk or other storage medium, or may be sent via satellite or cable, or over a network, such as the Internet. During the process of digital signal compression, redundant details are typically eliminated from the analog signal, allowing more data to be sent or stored.
Movies and television programs may be stored and transmitted in digital form. One advantage of storing and transmitting audio and visual information in digital form is that digital data does not become distorted as it is sent. Analog signals may pick up noise as they travel from a transmitter to people's homes. As previously mentioned, digitized movies and television programs may be stored on and played from a storage medium, or may sent via cable, satellite, or a network. It is possible for a user to download audio and visual data, including movies, from a network, such as the Internet.
When digitized data is to be viewed by a user, the digitized data must be converted back to analog audio and visual data. This process is referred to as decoding, and is performed by the decompression portion of a compression/decompression algorithm, referred to as a codec. A codec may be implemented in either hardware or software, although in most personal computers, codecs are implemented in software.
There are a number of compression technologies currently available. The Moving Pictures Experts Group, MPEG, is a group formed under the International Organization for Standardization, ISO, and is charged with the task of creating standards for digital signal compression. These standards are in the form of decoder specifications.
The first standard developed by MPEG was International Standard 11172 (ISO/IEC-11172), known as MPEG-1. MPEG-1 is generally considered to provide quality similar to VHS. It allows for the coding of audio/visual signals at a bit rate of approximately 1.5 megabits per second. The MPEG-1 standard is used in the movie industry for digitizing movies sold to customers on videodiscs. MPEG-1 is also used for transmitting audio and video data over telephone lines, which is how many people currently connect to networks, such as the Internet.
The next standard developed by MPEG was International Standard 13818 (ISO/IEC-13818), known as MPEG-2. MPEG-2 is generally considered to provide higher quality video at a faster bit rate of approximately 6 megabits per second. Users of the MPEG-2 standard include cable television companies, which provide cable television systems with hundreds of channels. MPEG-2 is a superset of MPEG-1, therefore an MPEG-2 codec is able to decode both MPEG-1 and MPEG-2 data.
The MPEG-1 specification defines a single system stream format, known as Systems Multiplex (SM). The MPEG-2 specification defines two system stream formats, known as the Program Stream (PS)and the Transport Stream (TS). MPEG-2 facilitates insertions in the Transport Stream format via "splice points," which allow additional digital data, such as commercials, to be added into the stream. However, there is no mechanism available to add additional data in either MPEG-1 Systems Multiplex or MPEG-2 Program Stream.
The codecs for MPEG-1 Systems Multiplex and for MPEG-2 Program Stream are usually simpler than the codecs for MPEG-2 Transport Stream. There are many applications using either MPEG-1 Systems Multiplex or MPEG-2 Program Stream, and yet these applications are not able to insert additional data into the stream data. Consequently, there is a need for a method and system which allow insertion of additional data into a digital audio/video data stream. It would be desirable to allow the inserted data to possess attributes different from those attributes found in the original data stream. It would also be desirable to allow the original data stream to continue playing (i.e. being decoded) without interruption after the inserted data has been played.